This invention relates to communication systems and more particularly to communication systems of the spread spectrum type.
The concept of spread spectrum as an anti-jamming technique is well known. Basically, it is desired to convey a narrow band intelligence, analog or digital in nature, occupying bandwidth w using a wideband noise-like carrier occupying bandwidth W. Both the transmitter and the receiver have copies of the noise-like carrier, but the jammer does not have a copy. The transmitter transmits the data by modulating the intelligence on the carrier using some modulation technique, such as phase shift modulation, frequency modulation, amplitude modulation, and the like. The receiver detects the intelligence by correlation detection. Because of this method of transmission, the effectiveness of a jammer will be reduced by the ratio of the spread spectrum bandwidth to intelligence bandwidth, that is, W/w. Thus, if the spread spectrum bandwidth is 100 times the intelligence bandwidth, a jammer with a given power P will have the effect of a noise-like jammer signal with P/100 applied to the intelligence signal being transmitted in a bandwidth w without spread spectrum techniques.
An essential part of this approach is that the jammer cannot reproduce the noise-like carrier and, thus, the correlation receiver rejects all but w/W of the jammer power because the noise-like carrier and the jammer's signal are independent. On the other hand, if the jammer can reproduce the noise-like carrier, it can generate a signal which consists of a narrow band modulation applied to the noise-like carrier which will be accepted by the receiver's correlator and can jam the desired signal to the same extent as if both the jamming signal and the desired signal had the same power in a narrow band w and spread spectrum were not used. In short, if the jammer can reproduce the noise-like signal, he can remove the spread spectrum advantage.
Typically, the noise-like carrier is a sine wave which is shifted in frequency at random among a set of n frequencies, of which is randomly phase shifted in steps of 0, 90, 180 or 270 degrees at some repetition rate. The frequency hopping equipment, or phase shifting equipment are driven by a digital pseudo-random (noise) code generator. Thus, the jammer's problem of generating the noise-like carrier used for transmission is largely that of generating the code. It is assumed that there is no possibility that a jammer can receive the desired signal, modify and amplify it, and retransmit it to the desired receiver so that the resultant signal is substantially accepted by the correlator. This technique is called repeat jamming and is often impractical. A typical procedure that a jammer might use to generate the code is the following. First, the generator obtains a long sequence of correct code digits. This code sequence can often be obtained from a receiver close to the transmitter. This receiver will have a strong signal and little jamming, if the jamming transmitter is far enough away. Alternatively, the jammer can silence his transmitter for some period. In many cases, the desired signal will then have a very good signal-to-noise ratio. Once a sequence of correct code digits is obtained, the jammer tries to break the code. After the code is broken, the jammer generates the noise-like carrier and the spread spectrum advantage is lost.
Present cryptographic codes and coding equipment for the United States are controlled by the National Security Agency (NSA). These codes and equipments are released for use only under stringent conditions imposed by NSA and only for specific tasks. On the other hand, the military services and agencies and even some commercial services would like to have spread spectrum capability in much of their communication equipment as a precautionary measure in case of intentional or unintentional jamming. Since the security requirements of cryptographic equipments are so much higher than those for spread spectrum systems, it has not provided feasible to supply coding equipment approved for cryptographic use with much of the spread spectrum equipment. Instead these spread spectrum equipments use some form of code generators, such as linear shift register generators, which are not approved for cryptographic use. It is well known, that once a small part of the code output of such code generators is known to the jammer he can easily break the whole code. A maximal length linear shift register generator with N stages will have a period of 2.sup.N -1 bits and can be broken rather easily once 2 N correct sequential bits are known. If N=100, the period is 2.sup.100 -1.apprxeq.10.sup.30 bits, and the code can be broken if 200 correct sequential bits of the code are given. If the spread spectrum code is broken, all the spread spectrum advantage which may be 20 to 30 db (decibel) is lost.
In summary, there is a problem in providing random code generators for spread spectrum systems. It is very expensive to provide cryptographically approved code generators with the appropriate security arrangements for all spread spectrum systems. On the other hand, the use of linear shift register codes, or similar "weak" codes for spread spectrum systems leaves these systems vulnerable to the loss of spread spectrum advantage that can result when a jammer breaks the code.
The problem with the use of the simple linear form of a linear shift register code is that the knowledge of a few correct code digits enables the breaking of the code with very little effort. Thus, there must be provided a way to complicate the form of the code to make it hard for the jammer to obtain a series of correct bits. Any attempt to complicate the code so that the code is secured, or any attempt to improve the difficulty of breaking the code immediately involves arrangements with highly classified cryptographic problems.